The present invention generally relates to the transport and culturing of cells.
The medical repair of bones and joints in the human body presents significant difficulties, in part due to the materials involved. Each bone has a hard, compact exterior surrounding a spongy, less dense interior. The long bones of the arms and legs, the thigh bone or femur, have an interior containing bone marrow. The material that bones are mainly composed of is calcium, phosphorus, and the connective tissue substance known as collagen.
Bones meet at joints of several different types. Movement of joints is enhanced by the smooth hyaline cartilage that covers the bone ends and by the synovial membrane that lines and lubricates the joint. For example, consider a cross-section through a hip joint. The head of the femur is covered by hyaline cartilage. Adjacent to that cartilage is the articular cavity. Above the articular cavity is the hyaline cartilage of the acetabulum which is attached to the ilium. The ilium is the expansive superior portion of the hip bone.
Cartilage damage produced by disease such as arthritis or trauma is a major cause of physical deformity and dehabilitation. In medicine today, the primary therapy for loss of cartilage is replacement with a prosthetic material, such as silicon for cosmetic repairs, or metal alloys for joint realignment. The use of a prosthesis is commonly associated with the significant loss of underlying tissue and bone without recovery of the full function allowed by the original cartilage. The prosthesis is also a foreign body which may become an irritating presence in the tissues. Other long-term problems associated with the permanent foreign body can include infection, erosion and instability.
The lack of a truly compatible, functional prosthesis subjects individuals who have lost noses or ears due to burns or trauma to additional surgery involving carving a piece of cartilage out of a piece of lower rib to approximate the necessary contours and insert the cartilage piece into a pocket of skin in the area where the nose or ear is missing.
In the past, bone has been replaced using actual segments of sterilized bone or bone powder or porous surgical steel seeded with bone cells which were then implanted. In most cases, repair to injuries was made surgically. Patients suffering from degeneration of cartilage had only pain killers and anti-inflammatories for relief.
Until recently, the growth of new cartilage from either transplantation or autologous or allogeneic cartilage has been largely unsuccessful. Consider the example of a lesion extending through the cartilage into the bone within the hip joint. Picture the lesion in the shape of a triangle with its base running parallel to the articular cavity, extending entirely through the hyaline cartilage of the head of the femur, and ending at the apex of the lesion, a full inch (2.54 cm) into the head of the femur bone. Presently, there is a need to successfully insert an implant device consisting of a macrostructure and a microstructure for containing and transporting cartilage cells and bone cells together with supporting nutrients, growth factors and morphogens, which will assure survival and proper future differentiation of these cells after transplantation into the recipient tissue defect. Presently, cartilage cells, called chondrocytes, when implanted along with bone cells, can degenerate into more bone cells because hyaline cartilage is an avascular tissue and must be protected from intimate contact with sources of high oxygen tension such as blood. Bone cells, in contrast, require high oxygen levels and blood.
Most recently, two different approaches to treating articular lesions have been advanced. One approach such as disclosed in U.S. Pat. No. 5,041,138 is coating bioderesorbable polymer fibers of a structure with chemotactic ground substances. No detached microstructure is used. The other approach such as disclosed in U.S. Pat. No. 5,133,755 uses chemotactic ground substances as a microstructure located in voids of a macrostructure and carried by and separate from the biodegradable polymer forming the macrostructure. Thus, the final spatial relationship of these chemotactic ground substances with respect to the bioresorbable polymeric structure is very different in U.S. Pat. No. 5,041,138 from that taught in U.S. Pat. No. 5,133,755.
The fundamental distinction between these two approaches presents three different design and engineering consequences. First, the relationship of the chemotactic ground substance with the bioresorbable polymeric structure differs between the two approaches. Second, the location of biologic modifiers carried by the device with respect to the device's constituent materials differs. Third, the initial location of the parenchymal cells differs.
Both approaches employ a bioresorbable polymeric structure and use chemotactic ground substances. However, three differences between the two approaches are as follows.
I. Relationship of Chemotactic Ground Substances with the Bioresorbable Polymeric Structure
The design and engineering consequence of coating the polymer fibers with a chemotactic ground substance is that both materials become fused together to form a single unit from structural and spatial points of view. The spaces between the fibers of the polymer structure remain devoid of any material until after the cell culture substances are added.
In contrast, the microstructure approach uses chemotactic ground substances as well as other materials, separate and distinct from the bioresorbable polymeric macrostructure. The microstructure resides within the void spaces of the macrostructure and only occasionally juxtaposes the macrostructure. Additionally, the microstructure approach uses polysaccharides and chemotactic ground substances spacially separate from the macrostructure polymer and forms an identifiable microstructure, separate and distinct from the macrostructure polymer.
The design and engineering advantage to having a separate and distinct microstructure capable of carrying other biological active agents can be appreciated in the medical treatment of articular cartilage. RGD attachment moiety of fibronectin is a desirable substance for attaching chondrocytes cells to the lesion. However, RGD attachment moiety of fibronectin is not, by itself, capable of forming a microstructure of velour in the microstructure approach. Instead, RGD is blended with a microstructure material prior to investment within macrostructure interstices and is ultimately carried by the microstructure velour.
II. Location of Biologic Modifiers Carried by a Device with Respect to the Device's Constituent Materials
Coating only the polymer structure with chemotactic ground substances necessarily means that the location of the chemotactic ground substance is only found on the bioresorbable polymeric structure fibers. The microstructure approach uses the microstructure to carry biologic modifiers such as growth factors, morphogens, drugs, etc. The coating approach can only carry biologic modifiers with the biodegradable polymeric structure.
III. Initial Location of the Parenchymal Cell
Because the coating approach attaches the chemotactic ground substances to the surfaces of the structure polymer and has no microstructure resident in the void volume of the device, the coating approach precludes the possibility of establishing a network of extracellular matrix material, specifically a microstructure, within the spaces between the fibers of the polymer structure once the device is fully saturated with cell culture medium. The coating approach predetermines that any cells introduced via culture medium will be immediately attracted to the surface of the structure polymer and attach thereto by virtue of the chemotactic ground substances on the polymer's surfaces.
The consequence of confining chemotactic ground substances to only the surfaces of the polymeric structure places severe restrictions on the number of cells that can be accommodated by the coated device. These restrictions on cell capacity are enforced by two limiting factors:
1) a severely limited quantity of chemotactic ground substance that can be incorporated within the device; and PA0 2) a surface area available for cell attachment that is limited by the surface area supplied by the structure polymer.
In contrast to the coating approach, the microstructure approach, by locating chemotactic ground substances in the void spaces of the device, makes available the entire void volume of the device to accommodate their chemotactic ground substance microstructure.